FIG. 35 is a cross section showing an example of a proximity sensor of a high-frequency oscillation type in the prior art. A conventional proximity sensor 101 includes a core 102 provided with an annular groove, in which a coil 104 held by a coil spool 103 is buried. Core 102 is held at a front surface of a coil casing 105 made of resin as shown in the figure, and these parts are accommodated in a base member, i.e., metal casing 106. A reference number 107 indicates an electronic circuit which includes an oscillator circuit including coil 104 as well as a signal processor unit for detecting lowering of its oscillation amplitude, and is mounted on a printed board 108. After coil 104 is connected to printed board 108, primary filler resin 109 is supplied into a portion of coil casing 105 near core 102 for stabilizing performance. In order to improve environmental resistance, epoxy resin 110 is supplied into the casing of the proximity sensor. When supplying epoxy resin 110, it is hot and, for example, an injector is used. A clamp portion 111 holds a code 112 to complete the proximity sensor. Alternatively, there has been a proximity sensor, in which a casing is not filled with resin in a sealed manner, and thermoplastic resin is used to form an integral structure. A display element 113 is mounted on printed board 108, and light beams emitted therefrom are led externally through a transparent light conducting portion 114.
For filling the casing of the proximity sensor with the resin, primary filler resin 109 of a low viscosity is injected into the coil casing 105, and coil 104 held by core 102 is inserted into the resin. Then, metal casing 106 is fitted to the coil casing 105, and epoxy resin 110 is injected again. In this conventional manner of filling the structure with resin, many steps are required for resin injection, and a long time is required for hardening.
The conventional proximity sensor requires a preliminary process such as stirring of resin to be injected. Even if the filler resin is injected into the casing with an injector or the like, it shrinks when hardened, so that additional resin must be injected to space formed by shrinkage. Therefore, many steps are required, and a long hardening time of about 1 hour is required. Further, in the case where thermoplastic resin is used for forming an integral structure, a high injection pressure is required for molding, so that accommodated parts may be impaired.
In the case where resin of a high viscosity is used as the filler resin for forming an integral structure, spaces around the coil casing, coil spool, core and others are not completely filled with the primary filler resin, and air remains in the primary molding dies. The remaining air expands due to a high temperature of the dies and a high temperature of the filler resin during the primary molding, and the coil casing may be deformed. The air remaining around the coil may cause unstable characteristics of the coil.
The present invention has been developed for overcoming the above disadvantages through low-pressure molding with thermoplastic resin, and has the following objects.
An object of the invention is to enable easy and reliable filling of a casing of an electronic component with resin. Another object of the invention is to simplify a resin filling process.